1. Field of the Invention
The present invention relates to a focus detecting system and, more particularly, to a focus detecting system which is adaptable to various types of imaging optical systems of different specifications to detect the focusing states thereof.
2. Description of the Prior Art
In single reflex camera systems or the like, the photographic lens assembly is interchangeable between those of different specifications. Therefore, a focus detecting system for such camera systems must be adaptable, in consideration of the fact that the photographic lens assembly is interchangeable between those of different specifications.
For example, it is not very practical to set the in-focus state at a single point to discriminate the in-focus state from the out-of-focus state. Therefore, it is the general practice to set a certain allowable range considering the depth of focus of the optical system involved, so that the in-focus state may be discriminated if the focusing state is within this allowable range. However, when this allowable range is too narrow, it is difficult to attain the in-focus state, resulting in inconvenience. To the contrary, when the allowable range is too wide, it becomes easier to attain the in-focus state while it becomes difficult to attain good focusing precision. In this way, ease in use and focusing precision essentially exclude each other. The balance between these two factors also varies according to the specifications (e.g., the F number, the focal length and so on) of the optical system used. For example, it is more practical to widen the allowable focusing range for an optical system having a greater depth of focus, while it is better to narrow the allowable focusing range for an optical system of a smaller depth of focus.
Apart from the point described above, when the focus detecting system is of the so-called "Through The Lens" type which has a light-receiving section to receive light transmitted by the optical system, the problems to be described below are caused by the various specifications of the optical system, especially by the focal length. If the size of the light-receiving section is fixed as the field of view of focus detection, it is too wide for a lens of short focal length, so that far objects and near objects may be mixed in the field of view. This causes the so-called "far-near competition" and resultant erroneous operation. To the contrary, if the angle of the field of view is too narrow for a lens of long focal length, only a limited part of the subject can enter the field of view for detection. This tends to result in that only a low contrast or solid pattern image enters the field of view. Thus it may become impossible to detect the focusing state. Furthermore, the image enters or leaves the field of view due to camera shake, resulting in unstable detection.